An electrophotographic technology has found widespread applications in the field of copying machines, and various printers because it can provide an image of immediacy and high quality.
As for the photoreceptor which is the core of the electrophotographic technology, photoreceptors using organic photoconductive materials having advantages of entailing no pollution, ensuring easy film-forming, being easy to manufacture, and the like, have been used.
As the photoreceptors using organic photoconductive materials, there are known a so-called dispersion type photoreceptor obtained by dispersing a photoconductive fine powder in a binder resin, and a lamination type photoreceptor obtained by laminating a charge generation layer and a charge transport layer. The lamination type photoreceptor has a high possibility of ranking as a dominant photoreceptor because a high sensitivity photoreceptor can be provided by using a charge generation material and a charge transport material each having a high efficiency in combination, a high safety photoreceptor can be obtained because of its wide material selection range, and it is advantageous in terms of cost due to its high productivity since a photosensitive layer can easily be formed by coating. Therefore, it has been vigorously developed and has gone into actual use.
The electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., in cycles of charging, exposure, development, transfer, cleaning, charge removal, and the like, during which it is subjected to various stresses to be deteriorated. Such deterioration includes chemical or electrical deterioration due to the following facts. That is, strongly oxidizing ozone or NOx arisen from, for example, a corona charger commonly used as a charger causes a chemical damage to a photosensitive layer, carriers (current) generated upon image exposure pass through the inside of the photosensitive layer, a photosensitive layer composition is decomposed by charge-removed light or light from the outside. Further, as other deterioration than such deterioration, there are mechanical deteriorations of abrasion or occurrence of flaws on the surface of the photosensitive layer, or peeling off of a film due to rubbing with a cleaning blade, a magnetic brush, or the like, contact with a developing agent or paper, and the like. Especially, such damage occurring on the photosensitive layer surface tends to become evident on the copied image. Accordingly, it directly damages the image quality and hence it is largely responsible for restricting the life of the photoreceptor. Namely, the enhancement of the electrical and chemical durability as well as the enhancement of the mechanical strength are essential conditions for developing a long-life photoreceptor.
In the case of a general photoreceptor having no functional layer such as a surface protective layer, it is a photosensitive layer that receives such a load. The photosensitive layer generally comprises a binder resin and a photoconductive material. It is the binder resin that substantially determines the strength. However, since the amount of the photoconductive material to be doped is considerably large, a sufficient mechanical strength has not yet been achieved.
Further, there has been a demand for a material adaptable to a higher-speed electrophotographic process to meet a growing need for a higher-speed printing. In this case, the photoreceptor is required not only to have a high sensitivity and a long life, but also to have good response characteristics so as to reduce the length of time between exposure and development thereof. It is known that, although the response characteristics are controlled by the charge transport layer, especially the charge transport material, it is also largely changed by the binder resin.
Each of the layers constituting the electrophotographic photoreceptor is formed by coating a coating liquid containing a photoconductive material, a binder resin and the like on a substrate by dip coating, spray coating, nozzle coating, bar coating, roll coating, blade coating, or the like. In such a method of forming layers, a known method of coating a coating liquid obtained by dissolving materials to be contained in the layer in a solvent, for example, has been applied. In many processes, a coating liquid is preliminarily prepared and preserved. Accordingly, the binder resin is required to be excellent also in solubility in a solvent used for coating process, and stability of the coating liquid after dissolution.
As the binder resins of the photosensitive layer, there have been used thermoplastic resins and various thermosetting resins, including vinyl polymers such as polymethyl methacrylate, polystyrene, and polyvinyl chloride, and copolymers thereof, polycarbonate, polyester, polysulfone, phenoxy, epoxy, and silicone resins. The polycarbonate resin has a relatively excellent performance out of a large number of the binder resins, and hence various polycarbonate resins have been developed and have gone into actual use so far (for example, JP-A-50-98332, JP-A-59-71057, JP-A-59-184251, JP-A-5-21478).
On the other hand, there is disclosed the technology of an electrophotographic photoreceptor using a polyarylate resin, commercially available under the tradename “U-polymer”, as a binder, and it is disclosed that the electrophotographic photoreceptor is particularly excellent in sensitivity as compared with the one using polycarbonate (for example, JP-A-56-135844).
Further, there is disclosed the technology of an electrophotographic photoreceptor using as a binder resin a polyarylate resin using a bivalent phenol component having a specific structure, and it has been known that the solution stability in manufacturing the photoreceptor improves and that the electrophotographic photoreceptor is excellent in the mechanical strength, especially the abrasion resistance (for example, JP-A-3-6567, JP-A-10-288845).
However, a conventional photoreceptor has such drawbacks that its surface is abraded by practical loads such as development by a toner, abrasion by paper and abrasion by a cleaning member (blade) or its surface may be scarred. Thus, in actuality, its printing performance is limited practically.